Ferrite thin film for high frequency and method for preparation thereof

ABSTRACT

The present invention provides a Y-type hexagonal ferrite thin film suitable for high frequency devices, having a crystal structure with the c-axis oriented perpendicular to the surface of the thin film. The present invention also provides a method of producing the Y-type hexagonal ferrite thin film, comprising the steps of preparing a viscous solution containing a metal-organic complex which is formed using a primary component including a Fe +3  ion, and a secondary component including a Ba 2+  ion, at least one transition metal ion selected from the group consisting of Fe 2+ , Co 2+ , Ni 2+ , Zn 2+ , Cu 2+  and Mn 2+ ; and optionally at least one metal ion selected from the group consisting of Sr 2+ , Ca 2+  and Pb 2+ , forming a film having a Y-type ferrite composition on a surface made of noble metal through a coating process using the viscous solution, and burning the film at a temperature of 750° C. or more.

TECHNICAL FIELD

The present invention relates to a ferroxplana-type hexagonal ferritethin film capable of obtaining a high magnetic permeability in the highfrequency range of high-frequency communication devices or the like, anda production method thereof.

BACKGROUND ART

Heretofore, cubic spinel-type ferrites as represented by Mn—Zn ferritehave been used in high frequency devices by taking advantage of theirhigh magnetic permeability. However, upon use in frequencies of severalhundred MHz, the permeability of the cubic spinel-type ferrites issharply deteriorated due to their Snoek's limit, and the effectivenessas material for high-frequency devices will disappear. Among hexagonalferrites, a ferroxplana-type ferrite with the c-plane having a highmagnetizability is expected as noteworthy material for high frequencydevices to be used in higher frequency range, because it can maintain ahigh magnetic permeability up to several GHz beyond the Snoek's limit ofcubic spinel-type ferrites. The ferroxplana-type ferrite has a typicalcomposition of Ba₂Zn₂Fe₁₂O₂₂ or Ba₃Co₂Fe₂₄O₄₁. There have also beenknown more complicated compositions such as a composition including SiO₂and CaO in addition to the above composition (Japanese Patent Laid-OpenPublication No. H09-129433) or Ba₃Co₂ (M_(x), N_(x)) Fe_(24-2x)O₄₁ (M:divalent metal ion such as Zn, Cu or Co; N: quadrivalent metal ion suchas Ti, Zr, Hf, Si, Ge, Sn or Ir; x: 3 or less; Japanese Patent Laid-OpenPublication No. 2000-235916). These hexagonal ferrites have been used aspowder material for a sintered body (Japanese Patent Laid-OpenPublication No. H09-129433) or powder paste for a coated layer (JapanesePatent Laid-Open Publication No. H09-205031).

Recently, in connection with advance of information and communicationsapparatuses such as portable phones and personal computers, downsizingand increase in signal frequency of electronic devices have beenaccelerated, which leads to the need for developing high-frequencyelectronic devices such as a filter or inductor available in higherfrequency range with more downsized structure. As a recent trend indownsizing, in view of the limit of a traditional approach of3-dimensionally downsizing a bulk device, a planer device effective todownsizing and integration is actively developed by utilizing atechnology of laminating thin films. However, despite of the strong needin small devices, no technology of forming a thin film using aferroxplana-type ferrite has been successfully developed.

As shown in FIG. 1, a hexagonal ferrite has M-type, U-type, W-type,X-type, Y-type and Z-type phases, and these phases have differentsolid-solution ranges, respectively. In addition, the crystal structurein each of the phases is extremely complicated as illustrated in FIG. 2.Thus, while there have been reported many cases of the formation ofM-type (BaFe₁₂O₁₉) thin films which is binary system and hasperpendicular magnetic anisotropy, none of the formation of other typehexagonal ferrite thin films has been reported. The M-type hexagonalferrite is a magnetoplumbite-type ferrite having uniaxial anisotropy,and is thereby used for quite different purposes from those of othertype hexagonal ferrites. Therefore, the need for developing a technologyof forming a Y-type hexagonal ferrite thin film usable in high frequencydevices strongly exists.

SUMMARY OF THE INVENTION

As a result of various researches for solving the above problem, theinventors has found that a thin film of Y-type hexagonal ferritebelonging to ferroxplana-type ferrites can be produced through a methodof forming a film on a noble metal surface by use of a viscous solutioncontaining a metal-organic complex.

Specifically, the present invention provides a Y-type hexagonal ferritethin film having a Y-type ferrite composition and a crystal structurewith the c-axis oriented perpendicular to the surface of the thin film,wherein the thin film is formed on the surface of a substrate made ofnoble metal, or the surface of a noble metal layer provided on asubstrate made of insulating or semiconducting material.

The present invention also provides a method of producing the aboveY-type hexagonal ferrite thin film. This method comprises the steps ofpreparing a viscous solution containing a metal-organic complex which isformed using a primary component including a Fe⁺³ ion, and a secondarycomponent including a Ba²⁺ ion and at least one transition metal ionselected from the group consisting of Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cu²⁺ andMn²⁺, forming a film having a Y-type ferrite composition on a surfacemade of noble metal, by use of the viscous solution, and burning thefilm. The secondary component may further include at least one metal ionselected from the group consisting of Sr²⁺, Ca²⁺ and Pb²⁺.

In this method, the preparing step may include the step of addingorganic acid and polyol to a solution of material prepared usingwater-soluble compounds containing Ba, Zn and Fe as starting materialsto form the metal-organic complex constituting the viscous solution,wherein the viscous solution is formed as a film having a composition ofY-type Ba₂Zn₂Fe₁₂O₂₂ on the surface made of noble metal and then burnt.

In the method, the burning step may be performed at a temperature of750° C. or more to obtain the above Y-type hexagonal ferrite thin film.

In the present invention, the composition of the Y-type hexagonalferrite thin film having a crystal structure with the c-axis orientedperpendicular to the surface of the thin film is typically expressed bya general formula Ba₂Me₂Fe₁₂O₂₂ (Me: at least one transition metalselected from the group consisting of Ni, Co, Zn, Cu and Mn).Alternatively, the Y-type hexagonal ferrite thin film may have acomposition formed by substituting Ba in the above basic compositionwith at least one metal selected from the group consisting of Sr, Ca andPb, or a more complicated Y-type hexagonal Ba ferrite composition formedby adding a small amount of various elements, such as B, Si or Mg, atany position in the above basic composition.

Heretofore, a solid-phase bulk Y-type ferrite has been burnt at about1000° C. By contrast, in the method of the present invention, a filmhaving a Y-type ferrite composition is formed on a surface made of noblemetal to allow a Y-type hexagonal ferrite thin film having a crystalstructure with the c-axis oriented perpendicular to the surface of thethin film to be obtained at a lower burning temperature. If the surfaceis made of Al₂O₃, the reaction between the ferrite and Al₂O₃ willpreclude any thin film formation. If the surface is made of quartz(SiO₂), no Y-type hexagonal ferrite thin film can be obtained because aformed film will be solid-solved in SiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of hexagonal ferrites.

FIG. 2 is a schematic diagram showing the atomic arrangement of a Y-typehexagonal Ba ferrite.

FIG. 3 illustrates a process of producing a tiny planar device using aY-type hexagonal ferrite thin film having a crystal structure with thec-axis oriented perpendicular to the surface of the thin film, whereinFIG. 3(A) is a top plan view, and FIG. 3(B) is a side view.

FIG. 4 is a graph showing the XRD pattern of a thin film produced inEXAMPLE 1.

FIG. 5 is a graph showing the magnetization curve of the thin filmproduced in EXAMPLE 1.

FIG. 6 is a photographic representation of the SEM image of the thinfilm produced in EXAMPLE 1.

FIG. 7 is a graph showing the XRD pattern of a thin film produced inEXAMPLE 2.

FIG. 8 is a graph showing the magnetization curve of the thin filmproduced in EXAMPLE 2.

FIG. 9 is a graph showing the XRD pattern on an M-type-Y-type line of athin film produced in COMPARATIVE EXAMPLE 1.

FIG. 10 is a phase diagram showing the composition of the thin filmproduced in COMPARATIVE EXAMPLE 1.

FIG. 11 is a graph showing the XRD pattern on a BaFe₂O₄-Y-type line of athin film produced in COMPARATIVE EXAMPLE 2.

FIG. 12 is a graph showing the Moessbauer spectrum on a BaFe₂O₄-Y-typeline of the thin film produced in COMPARATIVE EXAMPLE 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of producing a Y-type hexagonal ferrite thin film having acrystal structure with the c-axis oriented perpendicular to the surfaceof the thin film, according to the present invention, will now bedescribed in detail.

Firstly, a viscous solution containing a metal-organic complex formedusing a stating material capable of supplying a Fe⁺³ ion as a primarycomponent, another kind of divalent transition metal ion, a Ba²⁺ ion,and optionally a Sr²⁺, Ca²⁺ or Pb²⁺ ion is prepared. The term “viscoussolution” means a glutinous or sticky liquid, and more specifically anaqueous fluid in the form that a polymeric metal-organic complex istransparently and homogeneously dispersed in a solution such as water,acetic acid or ethanol.

This viscous solution may be prepared through (1) a process of slowlyconcentrating an initial water solution added with the starting materialwhile gradually vaporizing the water in the solution until it has adesired viscosity, or (2) a process of vaporizing the entire water in aninitial water solution added with the starting material to obtain a gelmetal-organic complex, and then dissolving the gel metal-organic complexin a solvent such as water, acetic acid, or ethanol to provide asolution having a desired viscosity.

As a specific example, a so-called metal complex polymerization processmay be preferably used to prepare the viscosity solution. In the complexpolymerization process, organic acid, such as citric acid, and diol,such as ethylene glycol, propylene glycol or butane diol, or any othersuitable polyol, are added to a water solution which contains a startingmaterial, such as metal carbonate, metal hydroxide, metal sulfate, metalcarboxylate or metal halogenide, dissolved therein to provide a givenferrite composition, and then the obtained mixture is dehydrated andcondensed to crosslink the metal ions with each other through theorganic matters so as to form an metal-organic complex.

The metal complex polymerization process itself is a known technologyfor homogeneously dispersing metal ions through complexifization of themetal ions using citric acid and formation of a 3-dimensional networkusing ethylene glycol. More specifically, a stable chelate complex isfirst formed between citric acid and plural kinds of metal ions. Then,the chelate complex is dispersedly dissolved in ethylene glycol, and theobtained solution is copolymerized and esterified under heating to forman oligomer and finally form a polymer gel having a 3-dimensionalnetwork structure or a complex polymerization. The complexpolymerization process is used in manufacturing a complex oxide such assuperconducting material.

The obtained polymer gel as a precursor has a significantly stablenetwork structure primarily formed through ester bonding orcopolymerization. Thus, the mobility of the metal ions is drasticallyreduced to allow ceramic particles to be formed directly therefrom witha desired composition in a subsequent process.

As compared to a sol-gel process using alcoxide as material, the metalcomplex polymerization process has advantages of enhanced homogeneity inmetal-ion dispersion, and lower cost in material. Further, when themetal complex polymerization process is used to produce a multinary thinfilm, the enhanced homogeneity in dispersion of metal-ion can beadvantageously maintained until a burning step by virtue of the3-dimensional network of the polymer. Any suitable conventional coatingor printing process, such as a dip coating or spin coating process, maybe used to form a film. Preferably, in each process of applying thepolymer gel, the thickness of a film to be applied is set in the rangeof about 80 to 100 nm. If it is required to increase the film thickness,the processes of film formation, drying, polymerization and thermaldecomposition may be repeated.

In the dip coating process, after immersion into a coating liquid, asubstrate is taken out of the coating liquid upward, and subjected to atreatment for gelatinizing the liquid film deposited on the surface ofthe substrate. The dip coating process has various advantages of no needfor expensive large-scale facility as in a chemical vapor depositionprocess, availability for any size of a substrate, simple operation, andapplicability in producing a multinary thin film.

As a pretreatment of the surface made of Ag, Au, Pt or other metal ofthe platinum family, on which a film having a composition of Y-typeferrite is formed, it is desired to rinse the surface of an oxide layerformed on the surface of the noble metal naturally or due to thermaloxidation, with a basic water solution such as KOH to modify the surfaceof the oxide layer completely by an OH group. Alternatively, the surfaceof the oxide layer may be simply rinsed with distilled water orhydrophilic liquid such as ethanol to obtain substantially the sameeffect. In case of Ag, even if its surface is briefly rinsed, it allowsa ferrite film to adequately deposited thereonto because an Ag oxidelayer is naturally formed thereon in the atmosphere. In case of Au orPt, almost no oxide layer is naturally formed on its surface. Thus, itis preferable to heat the surface under oxygen atmosphere or irradiatethe surface with oxygen plasma so as to positively form an oxide layeron the surface to provide sufficient wettability of a viscous solutionduring coating process or enhanced deposition of a film. The Ag, Au, Ptor other metal of the platinum family may contain any alloy element aslong as an intended function of its surface is not lost.

While the above noble metal has a cubic close-packed crystal structure,its surface can have an elongated directional microstructure due torolling or scratch. In this case, the crystal orientation in a Y-typehexagonal ferrite thin film can be enhanced by coating a film in adirection perpendicular to the direction of the microstructure ratherthan coating it in parallel thereto.

The conventional bulk device has a limit in 3-dimensional reduction insize, e.g. minimum thickness of about 3 mm or minimum radius of 2.5 mm,due to its structure composed of a plurality of separated components. Bycontrast, a Y-type hexagonal ferrite thin film of the present inventioncan be used to provide a device having a desired shape by coating aY-type ferrite film on a surface made of noble metal while placing onthe surface a mask with an opening corresponding to the desired shape,or by forming the Y-type hexagonal ferrite thin film over the surfaceand partially removing the thin film with etching to allow the remainingthin film to have the desired shape.

For example, as shown in FIG. 3, a significantly downsized planar devicehaving a thickness of about 20 μm and a radius of about 1 mm by forminga lower coil (a) having a surface layer made of one noble metal selectedfrom the group consisting of Ag, Au, Pt and other metal of noble metalfamily, on a substrate 1 made of insulating or semiconducting materialsuch as Al₂O₃ or Si, forming the Y-type hexagonal ferrite thin film (b)of the present invention on the surface layer, etching the thin film (b)to leave a ring-shaped thin film, and forming an upper coil (c) on thering-shaped thin film.

In the method of the present invention, when the film having the Y-typeferrite composition is burnt after formed on the surface made of noblemetal, the crystal orientation in a Y-type hexagonal ferrite thin filmto be obtained is deteriorated as the burning temperature is reduced.Thus, in either noble metal constituting the surface, the burningtemperature should be set at 750° C. or more to assure an enhancedin-plane crystal orientation in the obtained thin film. The burningtemperature of less than 750° C. precludes the formation of the intendedY-type hexagonal ferrite thin film having a crystal structure with thec-axis oriented perpendicular to the surface of the thin film, and thecrystal orientation will be enhanced as the burning temperature isincreased.

The upper limit of the burning temperature is preferably set at 900° C.for the surface made of Ag (melting point: 960.8° C.), or at 1300° C.for the surface made of Pt (melting point: 1769° C.), or at 1000° C. forthe surface made of Au (melting point: 1063° C.). In the surface made ofAg or Au, upon exceeding the upper limit, the burning temperature getsclose to their melting point, and the smoothness of the obtained thinfilm is sharply deteriorated. In the surface made of Pt, at atemperature exceeding the upper limit, the ferrite itself in the coatedfilm is partially molten and decomposed. It is particularly preferableto use Ag as the material of the surface, because the burningtemperature for the surface made of Ag is set less than the meltingpoint (962° C.) of a silver wire for use as electrical leads to allowsilver wire to be burned together with the coated film. In view of thesituation where high frequency devices generally employ a silver wire intheir circuit, the Y-type hexagonal Ba ferrite thin film formed on thesurface made of Ag is a significantly effective material for highfrequency devices.

EXAMPLE Example 1

Starting materials, BaCO₃, ZnCO₃ and FeCl₃.6H₂O were weighted to havetheir ratio of 1:1:6, and put in a beaker. On the basis of the total molnumber of the starting materials, 70-time mol of distilled water,3.75-time mol of citric acid, and 11.25-time mol of ethylene glycol wereadded to the starting materials to form a metal complex.

The obtained viscous solution was heated at 90° C. by a hot stirrer, andconcentrated. After the stirrer is completely stopped, the obtained gelwas diluted by adding 2 parts of acetic acid on the basis of the weightof the gel. A pure Ag substrate (0.20×10×10 mm, available from NilacoCo.) is dipped in the viscous solution prepared in the above way, andtaken out of the viscous solution upward at a constant speed to form afilm coated on the surface of the Ag substrate. The coated film wassubjected sequentially to drying at 90° C. for 1 hour, polymerization at190° C. for 1 hour, and thermal decomposition at 410° C. for 1 hour. Inthe above process, when the substrate was taken out of the viscoussolution upward at a constant speed of 43.7 mm/min, a coated film had athickness of about 1000 Å. Then, the substrate with the coated film wasburnt at various temperatures.

The obtained thin film was evaluated by identifying the created phaseusing XRD, measuring the magnetization using VSM, observing the surfaceof the thin film using SEM, determining the magnetization phase throughMoessbauer spectroscopy, and measuring the thickness of the thin film.The XRD pattern of the produced thin film is shown in FIG. 4. A Y-typepeak appeared in the thin film burnt at 900° C. for 1 hour. In addition,despite of Ag having a cubic crystal structure, all of the obtainedY-type peaks were oriented in the direction of (001) plane.

The magnetization curves of the above thin film are shown in FIG. 5. Ascompared to the hysteresis curve obtained by applying a magnetic fieldin the vertical direction of the thin film, the hysteresis curveobtained by applying a magnetic field in the in-plane direction of thethin film has a sharper rising, and consequently a smaller coercivity.That is, the produced thin film has high in-plane magnetizability. TheSEM image of the thin film is shown in FIG. 6. Hexagonal plate-shapedgrains observed in FIG. 6 prove the creation of a hexagonal crystalstructure.

Example 2

A thin film was produced under the same conditions as those in EXAMPLE 1except for using a pure Pt substrate (0.20×10×10 mm, available fromNilaco Co.) and coating a film on the surface of the Pt substrate. TheXRD pattern of the produced thin film is shown in FIG. 7. When a surfacemade of Pt was used, a Y-type peak appeared in the produced thin filmburnt at 1100° C. for 1 hour. In addition, despite of Pt having a cubiccrystal structure, all of the obtained Y-type peaks were oriented in thedirection of (001) plane.

The magnetization curves of the above thin film are shown in FIG. 8. Ascompared to the hysteresis curve obtained by applying a magnetic fieldin the vertical direction of the thin film, the hysteresis curveobtained by applying a magnetic field in the in-plane direction of thethin film has a sharper rising, and consequently a smaller coercivity.That is, the produced thin film has high in-plane magnetizability.

Comparative Example 1

A thin film was produced under the same conditions as those in EXAMPLE 1except for using a α-Al₂O₃ c-plane substrate and coating a film on thesurface of the Al₂O₃ substrate. A coated film was burnt at 1100° C. for30 minutes. The XRD pattern on an M-type-Y-type line of the producedthin film is shown in FIG. 9, and the composition of the produced thinfilm is plotted on a phase diagram in FIG. 10. When α-Al₂O₃ c-plane wasused as a surface, only peaks of the thin film reacted with thesubstrate appeared. While an M-type Ba ferrite phase could be observedin all of the compositions of the produced thin film as shown in FIG. 9,no Y-type phase could be created.

Comparative Example 2

In COMPARATIVE EXAMPLE 1, there is a possibility that Zn having a lowmelting point was released outside the phases. Thus, a coated film underthe same conditions as those in EXAMPLE 1 was burnt at a lowertemperature of 1000° C. In this case, a previously unknown phaseappeared. In view of the peak of this unknown phase exhibited at aposition close to a hexagonal α-Al₂O₃ substrate and the burningtemperature, it can be assumed that the unknown phase is also hexagonalα′-BaFe₂O₄. The XRD pattern on a BaFe₂O₄-Y-type line of the producedthin film is shown in FIG. 11. As seen in FIG. 11, an unknown phaseappeared in the composition having Zn. FIG. 12 is the Moessbauerspectrum on a BaFe₂O₄-Y-type line of the produced thin film. A peak inthe composition of BaFe₂O₄ shows only an antiferromagnetic phase, whichresults from α′-BaFe₂O₄.

However, the peak of the antiferromagnetic phase goes down and the peakof a paramagnetic phase goes up as the composition gets close to Y-type.It is believed that the paramagnetic phase is a compound (AlFeO₃)similar to an ilmenite structure created through the reaction between Fein the components of the coated film and Al in the components of thesubstrate. Through this example, it was proved that the intended thinfilm cannot be obtained using the α-Al₂O₃ substrate due to the reactionbetween the coated film and the substrate.

INDUSTRIAL APPLICABILITY

The Y-type hexagonal ferrite thin film of the present invention isuseful as material for high-frequency electronic devices such as afilter or inductor available in higher frequency range.

1. A Y-type hexagonal ferrite magnetic thin film composite, comprising:a thin film of in-plane magnetizability including Y-type ferrite, c-axisof crystal structure of said Y-type ferrite being oriented perpendicularto a surface of said thin film, and a substrate with a surfaceconsisting of noble metal, said thin film being directly formed on thesurface consisting of noble metal.
 2. The Y-type hexagonal ferritemagnetic thin film composite as defined in claim 1, wherein thesubstrate with a surface consisting of noble metal comprises a substratemade of noble metal.
 3. The Y-type hexagonal ferrite magnetic thin filmcomposite as defined in claim 1, wherein the substrate with a surfaceconsisting of noble metal comprises: a substrate made of insulating orsemiconducting material; and a noble metal layer provided on thesubstrate made of insulating or semiconducting material.
 4. A highfrequency device comprising: a Y-type hexagonal ferrite magnetic thinfilm composite comprising: a thin film of in-plane magnetizabilityincluding Y-type ferrite, c-axis of crystal structure of said Y-typeferrite being oriented perpendicular to a surface of said thin film, anda substrate with a surface consisting of noble metal, said thin filmbeing directly formed on the surface consisting of noble metal.
 5. Amethod of producing the Y-type hexagonal ferrite thin film composite asdefined in claim 1 or 4, comprising the steps of: preparing a viscoussolution containing a metal-organic complex which is formed using aprimary component including a Fe⁺³ ion, and a secondary componentincluding a Ba²⁺ ion and at least one transition metal ion selected fromthe group consisting of Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cu ²⁺ and Mn²⁺; forminga film having a Y-type ferrite composition on a surface made of noblemetal, by use of said viscous solution; and burning said film.
 6. Themethod as defined in claim 5, wherein said secondary component furtherincludes at least one metal ion selected from the group consisting ofSr²⁺, Ca²⁺ and Pb³⁺.
 7. The method as defined in claim 5, wherein saidpreparing step includes the step of adding organic acid and polyol to asolution of material prepared using water-soluble compounds Ba, Zn andFe as a starting material to form said metal-organic complexconstituting said viscous solution, wherein said viscous solution isformed as a film having a composition of Y-type Ba₂Zn₂Fe₁₂O₂₂ on saidsurface made of noble metal and then burnt.
 8. The method as defined inclaim 5, wherein said burning step is performed at a temperature of 750°C. or more.